Okay, well for my first playthroughs, I'd pick the starter that had the most badass-yet-not-funky-looking final EVO out of the fire/water starters. Grass just never appealed to me for some reason. Anyway, so this is how it went down:
Yellow: Pikachu, of course.
Blue: Squirtle!
Silver & Crystal: Cyndaquil
Sapphire: Torchic (Though I kind of regretted it after I saw what my cute little chicky pokemon evolved into)
Ruby: Mudkip (Swampert wasn't very good-looking, either, but I still didn't want to choose Treeko)
LG: I picked Squirtle because that was what I had in my Blue Version.
FR: I picked Charmander because it was FIRE RED and I felt it was fitting.
Pearl: Piplup, because everyone else was choosing Chimchar and I wanted to try something different.
Well when I first started pokemon, I would level up my starter to a ridiculously high level and keep my other pokemon pretty much random and I used them whenever I felt like it. I remember my team in Yellow was something like this:
Pikachu, level 65 and Blastoise, level 100, and some other fodder pokemon including Hitmonchan, because I recently came across a "journal" entry about my pokemon adventure that I had written at the time. LOL. Anyway, after teaching my Blastoise Ice Beam from that TM the vending machine girl gave me, I think I just went crazy and defeated the E4 several times.
Then in Silver, they added genders, and I was pleased to find out that my starter, Cyndaquil, was a female. At that time I had no idea about gender ratios, but at the time somebody told me that female pokemon were stronger. Then I went through my journey and caught a Mareep, and I remember evolving my Togepi and Eevee really "early" at like levels 9 and 29, respectively, and my friends were all jealous. Then I leveled my Typhlosion up to level 100 and it had the moveset Flamethrower/Earthquake/ThunderPunch/Cut and it was UNSTOPPABLE. And I had a level ~70 Espeon. That was my favorite generation by far.
Then when RSE came out I grasped the concept of leveling up evenly and I trained my team accordingly with a decent balance of types. My Sapphire team was something like Minun, Ninetales, Brelloom, Altaria, Kyogre, and Sharpedo. But after reaching the E4 underleveled Kyogre basically dominated and ended up like 20 levels above the rest of my pokemon. Same thing basically happened in my Ruby version.
Then I didn't enjoy my FRLG experience very much, perhaps because the games were too similar to RBY, and I consequently do not remember much about them and I played through my Fire Red several times.
Then I finally got Pearl, and I got the other two starters I had missed from the GTS, and then I finally realized the importance of natures and after beating the game, I started breeding.
One thing I've noticed is that I've grown quite attached to the pokemon in my first playthrough for many of my games and I don't have the heart to start a new game. Maybe it's just the feeling of beating the Elite Four hundreds of times that makes you not want to start over, even though my pokemon have crappy natures and probably worse IVs. But I'm still attached to them and I haven't released them despite my great need for box space.
Well that was a long-winded speech but if you want to read the important part of this post:
Well, before I didn't care about leveling up evenly, or Natures. Now I go around SRing (so I don't waste pokeballs) for good-natured pokemon, which incidentally takes a lot of time. I also feel the need to level them up evenly, because it really bothered me when I needed to use a grass pokemon and my best one was like 30 levels weaker than my strongest pokemon. But all of this takes quite a lot out of the game, but I can't bring myself to play in the way I used to because I keep thinking, "My pokemon could have been so much stronger if it were Adamant (or other nature)!"
I haven't really tried any mono-type playthroughs or anything but I may consider it in the future, although what I really want is just to play the game without caring about natures or IVs or EVs or anything and just playing like I did before, when I was ignorant of all of this stuff. It was just more fun.
/endrant